1. Field of the Invention
The present invention relates to an ionization apparatus and ionization method for mass spectrometry, and more particularly relates to an apparatus and method for ionization using an ion trap type ion source and metal ion attachment method in ion mass spectrometry.
2. Description of the Related Art
A mass spectrometry apparatus generally introduces a sample gas including a target substance for analysis, ionizes the sample gas, separates and takes out the ions relating to the target substance for analysis from the ions not analyzed, and measures and analyzes the mass. The mass spectrometry apparatus includes an ion source for ionization of the sample gas and a mass spectrometry unit for measuring and analyzing the ions desired to be analyzed. An ion trap type mass spectrometry apparatus can trap ions of a specific mass by a structural unit serving both as the ion source and mass spectrometry unit, so alternately ionizes the gas and analyzes the mass.
There are various methods for ionization of a sample gas in an ion source of a mass spectrometry apparatus. In the electron impact method (EI), electrons are fired at an ionization region into which only the sample gas is introduced so as to directly ionize the sample gas. Further, in the chemical ionization method (CI), electrons are fired into an ionization region where a reaction gas including a small amount of the sample gas is introduced and the reaction gas ionized. Next, the ionized reaction gas (reaction ions) is reacted with the sample gas and the H+ in the reaction ions attached to the sample gas to ionize the sample gas.
In the chemical ionization method, the above ion trap type mass spectrometry apparatus traps the reaction ions, so the opportunities for impact between the reaction ions and sample gas increase. Therefore, compared with an ordinary two-dimensional Q-pole (quadrapole) type mass spectrometry apparatus which does not trap ions, there is the advantage that use is possible at a low pressure.
An ion trap type mass spectrometry apparatus has a structural unit serving as both an ion source and mass spectrometry unit, but a mass spectrometry apparatus of a type with these as independent separate structural units configured so as to use the above characterizing structural unit positively as an ion source has also been proposed (Japanese Patent No. 2679026). In this patent, electron impact ionization or chemical ionization is used as the ionization method in the ion source. The conventional ion source described in that patent is configured to switch between electron impact ionization and chemical ionization as the ionization method. It does this just by changing the parameter of the alternating current or direct current applied to the structural unit, so does not use a mechanical switching operation. Further, in the ionization, ions of all of the ingredients of the sample gas are produced, so only the ions desired to be measured and analyzed are roughly separated and ejected to the mass spectrometry unit. By changing the parameter to control the stable state and unstable state of the ions, the ions desired to be analyzed are ejected to the Z-direction (axial direction of electrode unit) and the ions not analyzed are scattered in the R-direction (radial direction of electrode unit) so as to roughly separate the ions.
Further, one of the methods of ionization is the metal ion attachment method. The metal ion attachment method uses the property that for example the Na+ or other metal ions emitted from the ion emitter gently attach to and ionize the gas molecules in their original form. According to the metal ion attachment method, the production of low molecular weight substances by the disassociation of the sample gas is suppressed and efficient ionization becomes possible.
In an apparatus making positive use of the ion trap structural unit as an ion source such as the mass spectrometry apparatus disclosed in the above Japanese Patent No. 2679026, when using the chemical ionization method, the chemical ionization method (1) produces not the H+ finally added as the reaction ions to react with the sample gas, but CH5+ or C2H6 including the same and (2) adds hydrogen to the molecules contained in the sample gas in the reaction, so shifts the ingredients included in the sample gas in mass by exactly 1 amu (atomic mass unit). Therefore, the following problems arise with respect to the separation of the ions desired to be analyzed and the ions not analyzed.
The problem at the time of chemical ionization in a mass spectrometry apparatus using an ion trap structural unit as an ion source is that a high resolution is required for separating [1] the ions to be scattered and extinguished inside the structural unit, [2] the ions to be trapped (reaction ions), and [3] the ions to be ejected to the mass spectrometry unit (ions of target substance for analysis). Further, some ions are not accurately separated even with a high resolution. This will be explained with reference to FIG. 7 and FIG. 8. In FIG. 7 and FIG. 8, the abscissas indicate the mass number, while the ordinates indicate the intensity. These figures show the distribution of the ingredients of the sample gas or the reaction ions or other gases or ions on the abscissas of the mass number.
FIG. 7 is a view for explaining the state of separation of the ions of the above [2] and [3], that is, the ions to be scattered and extinguished inside the structural unit since they are unnecessary for analysis and interfere with the mass spectrometry and the reaction ions to be trapped inside the structural unit for efficient reaction with the target substance for analysis (gas ingredients of sample gas). In FIG. 7, when the reaction gas is methane CH4 (mass 16) 101 as shown in the top graph (A), if an electron beam is fired, as shown by the bottom graph (B) of FIG. 7, a plurality of ions (CH+, CH2+, CH3+, CH4+, CH5+, C2H3+, C2H4+, C2H5+, C3H3+, C3H4+, C3H5+ and C3H7+) are produced. Among these ions, the CH5+ (102) or C2H5+ (103) shown by the hatching in the figure are reaction ions reacting with the sample gas. The other ions shown by the regions 104, 105, and 106 are unnecessary and rather interfere with the mass spectrometry, so should be scattered and extinguished inside. In the case of the above example, the reaction ions have to be trapped in the ion source, while the other ions have to be scattered and extinguished inside the container of the ion source.
As clear from (B) of FIG. 7, since the ions to be scattered and extinguished inside adjoin the reaction ions to be trapped (CH5+ or C2H5+) at the low mass side, it is difficult to separate the two. This requires a hardware configuration having a high resolution, that is, a high precision structural unit and a high degree of voltage control with respect to the structural unit. Further, if set to trap specific ions for separation, the ions on the low mass side from the trapped ions are scattered and extinguished and the ions on the high mass side are ejected to the mass spectrometry unit (the movement of the low mass side and high mass side can also be reversed). Therefore, ions (ions of region 106) are also present at the high mass side of the reaction ions 102 and 103 trapped, but it is impossible to extinguish these.
FIG. 8 is a view for explaining the state of separation of ions of the above [2] or [3] emitted to a mass spectrometry unit for analysis at the mass spectrometry unit. In FIG. 8, the top graph (A) shows two types of reaction ions CH5+ (201) and C2H5+ (202), the middle graph (B) shows the analyzed gas before ionization, and the bottom graph (C) shows the analyzed gas and reaction ions ionized by chemical ionization. The analyzed gas includes as ingredients at least C, CH4, H2O, HF, C2, C2H4, and C2H6. If this analyzed gas is ionized by chemical ionization, as shown by the bottom graph (C), the hydrogen ions in the reaction ions are added for a shift of exactly 1 amu to generate ions of CH+, CH5+, H3Oxe2x88x92, H2F+, C2H+, C2H6+, and C2H7+. Further, in the bottom graph (C), the reaction ions CH5+ (201a) and C2H5+ (202a) shown by the hatching are the trapped ions. The other ions, that is, the ions included in the regions 203, 204, and 205, are ions ejected to the mass spectrometry unit.
As clear from the array of ions in the bottom graph (C) of FIG. 8, the ranges of the ions to be trapped and the ions to be ejected to the mass spectrometry unit are substantially the same, so a hardware configuration having a high resolution becomes necessary to separate the two. Further, since the ions on the low mass side from the trapped ions are scattered and extinguished, 203 cannot be analyzed in the case of CH6 and 203 and 204 cannot be analyzed in the case of C2H6. Further, cases arise of complete superposition. In this case, separation becomes impossible.
An object of the present invention is to solve the above problem and provide an ionization apparatus and ionization method which enable accurate separation of the ions desired to be analyzed and the ions desired to be trapped in mass spectrometry by a simple configuration and relatively low resolution and improve the sensitivity of analysis.
To achieve the above object, the ionization method and ionization apparatus for mass spectrometry according to the present invention are comprised as follows:
The first ionization apparatus is applied to a mass spectrometry apparatus using an ion trap type structural unit as an ion source and is provided with an ion emitter for emitting metal ions inside or outside of the ion source. The metal ions emitted from the ion emitter are attached to the ingredients of the sample gas so as to ionize the sample gas, the preset separation parameter is changed to separate the ions relating to the target substance for analysis and the metal ions, the metal ions are trapped and accumulated inside the ion source, and the ions relating to the target substance for analysis are ejected to the mass spectrometry unit.
According to the above ionization apparatus, by configuring the ion source by combining an ion trap type unit and metal ion attachment method, it is possible to increase the difference of atomic mass units between the ions desired to be analyzed and the ions desired to be trapped at the time of ionization and thereby possible to simply and accurately separate the ions (metal ions) desired to be analyzed by a hardware configuration having a relatively low resolution.
A second ionization apparatus comprises the above configuration wherein further the ion trap type structural unit is comprised of a ring-shaped electrode and two end gap electrodes.
A third ionization apparatus comprises the above configuration wherein further the ring-shaped electrode has a cylindrical shape and the two end gap electrodes have disk shapes. Since it is possible to easily separate the ions of the target substance for analysis, it is possible to make the shape and structure of the electrode portion simpler.
A fourth ionization apparatus comprises the third configuration wherein further an insulator is provided between the ring-shaped electrode and two end gap electrodes; the sample gas is directly introduced inside the ion source; the ion emitter is provided outside the ion source; and an ionization chamber in which the ion source is placed is evacuated so that a pressure outside the ion source becomes lower than a pressure inside it. According to this configuration, since the pressure outside the ion source becomes lower than the pressure inside, it is possible to prevent contact of the sample gas with the ion emitter as much as possible. Due to this, it is possible to prevent the ion emitter from contamination by the sample gas and possible to extend the service life. Note that in the above configuration, the metal ions emitted from the ion emitter are introduced inside the ion source by an electric field generated by the attached electrodes.
A first ionization method is a method for mass spectrometry using an ion trap type structural unit as an ion source comprising generating metal ions, attaching the metal ions to the ingredients of the sample gas to ionize the sample gas, changing the separation parameter to separate the ions relating to the target substance for analysis and metal ions, trapping and accumulating the metal ions inside the ion source, and ejecting the ions relating to the target substance for analysis to a mass spectrometry unit.
According to the present invention, since the sample gas is ionized by the metal ion attachment method in a mass spectrometry apparatus using an ion trap type structural unit as an ion source and provided with a metal ion emitter, it is possible to accurately separate ions desired to be analyzed and trapped ions by a simple configuration and relatively low resolution. Further, since a large amount of the trapped ions, that is, metal ions, is accumulated, it is possible to improve the sensitivity of the mass spectrometry. Further, since the sample gas does not contact the ion emitter, it is possible to prevent contamination of the ion emitter and extend the service life.